Abstract

We demonstrate a diffractive maskless lithographic system that is capable of rapidly performing both serial and single-shot micropatterning. Utilizing the diffractive properties of phase holograms displayed on a spatial light modulator, arbitrary intensity distributions were produced to form two and three dimensional micropatterns/structures in a variety of substrates. A straightforward graphical user interface was implemented to allow users to load templates and change patterning modes within the span of a few minutes. A minimum resolution of ~700 nm is demonstrated for both patterning modes, which compares favorably to the 232 nm resolution limit predicted by the Rayleigh criterion. The presented method is rapid and adaptable, allowing for the parallel fabrication of microstructures in photoresist as well as the fabrication of protein microstructures that retain functional activity.

Figures (4)

The SLM-based diffractive patterning system. The output from a Q-switched 532 nm laser is polarized, collimated, and expanded to address the display of a SLM operating in phase-only mode. Three relay lenses and several mirrors direct the SLM modulated beam to the pupil plane of a high NA microscope objective. (a) Close up of an intensity distribution generated for single-shot processing, three shapes are projected into a sample material at once. (b) An intensity distribution of nine foci generated for the simultaneous serial processing of nine unique structures. (c) Close up of the SLM display showing the expanded, collimated beam incident upon a phase hologram. The phase hologram can be modified to produce any desired intensity distribution including those seen in (a) and (b).

SEM images of various 2D patterns fabricated using the described method with a 100x objective. The inset SEM images captured at a 26° angle to the sample surface show the photoresist depth. The patterns in (a-e) were fabricated using the single-shot mode, while the pattern in (f) was fabricated in serial mode. As shown in movie [(a) Media 1], single-shot patterns required the display of 30 phase holograms, during a 10 second fabrication time at an average laser power of ~1 mW, to eliminate speckle and produce smooth continuous features. The arrays and arbitrary shapes demonstrate the versatility of the single-shot method. The array in (f) shows the ability to provide high-throughput processing in serial mode, while the inset displays replication accuracy. The phase holograms were displayed on the SLM at 8 fps and created 25 independent foci to simultaneously transfer each feature of the 5 x 5 array. An average power of 1 mW was used. Scale bars are 5µm except the inset of (f) which is 20 µm.

SEM micrographs of 3D microstructures fabricated using the serial processing mode. (a) The 9-feature pattern was designed in CAD software and then all features were simultaneously fabricated in NOA 63 using a single phase-modulated source beam (Media 2). The various views (b-d) show the morphology of the pattern. Features 2, 5, 7, & 9 are 5 µm in height, features 1, 4, & 8 are 7.5 µm in height, and features 3 & 6 are 10 µm in height. The stars in each image indicate the pattern orientation. The holograms were displayed on the SLM at 8 fps with an average power of 62 mW through a 100x objective. (e-h) A bridge type structure fabricated in SU-8 2010 demonstrates complete 3D control of fabrication, including the ability to create voids within the microstructure. Holograms were displayed on the SLM at 2 fps with an average power of 62 mW through the 100x objective. The scale bars in all images are 5 µm.

BSA-FITC solution exposed to three time-averaged patterns simultaneously at 110 mW for 10 s. (a) Real-time microscope image captured during fabrication. (b) Fluorescent image of patterned protein after sample wash with 1x PBS. (c) SEM image of sample after a chemical desiccation process. Predicted feature dimensions were assessed using area measurements from both the fluorescent and SEM images. The fluorescent measurements indicated the features were ~6% larger than desired, while the SEM measurements indicated they were ~6% smaller than desired; demonstrating an accurate tolerance for pattern transfer. Next, a BSA-FITC solution spiked with avidin conjugated FITC was patterned. Biotin conjugated with ATTO 655 was applied to the resulting BSA/avidin structures for 20 min to assess the retention of avidin binding ability. Fluorescent microscope images were taken using (d) FITC and (e) Cy5 filter sets. A combined fluorescent image (f) reveals the functionality of the substrate after patterning. All scale bars are 50 µm.